In the rat spermatogenesis is organized into a series of 14 stages which are defined by the observed types of germ cell associations. In the normal rat, all 14 stages are represented in a frequency which is proportional to their duration. Germ cells present in one stage simultaneously develop into the cells which define the subsequent stages. Because spermatogenesis is asynchronous, the dynamics of this cycle of the seminiferous epithelium provide a continuous source of sperm in most mammals. Sertoli cells are associated with the germ cells in each stage of the cycle nd undergo cyclic variations in their functional parameters. When rats are subjected to a vitamin A deficient die for 8 to 9 weeks and then injected with retinol the seminiferous epithelium undergoes an initial regression followed by a recovery. When spermatogenesis is reinitiated with retinol only a few closely related stages of the cycle are present. These stages progress through the cycle normally and the testes are synchronized. This model system provides a number of advantages in the study of the cycle of the seminiferous epithelium. An entire testis consists of only a new stages of the cycle so tissue representing a region of the cycle is possible. Only limited sub-types of germ cells uncontaminated by germ cells in other parts of the cycle are present in the synchronized testis. The specific aims of the next grant period involve continued characterization of the mechanisms regulating cyclic expression of selected gene products in the testis. We propose to determine the maintenance or loss of stage specific gene expression in tubules and cells in culture and to ascertain whether changes in mRNA levels across the cycle are due to changes in transcription or mRNA stability. We will examine the kinetics and testicular distribution of the onset of asynchrony in stage synchronized testes. We will utilize the advantages of the synchronized testis system to determine the parts of the cycle which are most susceptible to hormone regulation and to obtain specific molecular markers for subpopulations of germs cells. The specific aims are all related in that they are designed to ask specific questions about the cycle or they will provide data which allow for more difficult questions to be asked in the future.